1. Field of the Invention
The present invention relates to a semiconductor device manufacturing method using laser light illumination which method is superior in mass-productivity and can provide small variations and a high yield, and to a laser processing apparatus usable in such a method. In particular, the invention relates to a method and an apparatus for improving or recovering the crystallinity of a semiconductor material part or all of which is made of an amorphous component, a substantially intrinsic polycrystalline semiconductor material, or a semiconductor material which crystallinity has been greatly degraded due to irradiation of ions, ion implantation, ion doping, or the like, by illuminating such a semiconductor material with laser light.
2. Description of the Prior Art
In recent years, extensive studies have been made to lower the temperature of semiconductor manufacturing processes. This is due to the need of forming semiconductor devices on an insulative substrate, such as a glass substrate, that is not highly heat resistant, as well as to miniaturization and multi-layering of devices.
In semiconductor processes, it is sometimes necessary to crystallize an amorphous component included in a semiconductor material or an amorphous semiconductor material, cause a semiconductor material to restore its crystallinity that has been degraded due to irradiation with ions, or improve the crystallinity of a crystalline semiconductor material.
Conventionally, thermal annealing is used for such purposes. When silicon is used as a semiconductor material, crystallization of an amorphous material, recovering and improvement of the crystallinity, and the like are performed by annealing of 0.1 to 48 hours or longer at 600 to 1,100xc2x0 C.
In general, as the temperature is increased, the thermal annealing can be performed in a shorter period and becomes more effective. At a temperature lower than 500xc2x0 C., the thermal annealing causes almost no effects. Therefore, from the viewpoint of lowering the process temperature, it is necessary to replace a conventional manufacturing step involving thermal annealing with a step using some other means.
An annealing technique using laser light illumination now attracts much attention as an ultimate low-temperature process to replace the thermal annealing. This is because laser light can be applied only to a portion that requires high energy equivalent to that of the thermal annealing and, therefore, it is not necessary to expose the entire substrate to a high-temperature environment.
Generally, there have been proposed two methods of laser light illumination. In the first method, a CW laser such as an argon ion laser is used to apply a spot-like beam to a semiconductor material. After being melted, the semiconductor material is gradually solidified and thereby crystallized due to an uneven energy profile of the beam and movement of the beam.
In the second method, a pulsed laser such as an excimer laser is used to apply a high-energy laser pulse to a semiconductor material. The semiconductor material is crystallized by instantaneously melting and then solidifying it.
The first method has a problem of long processing time. This is because a CW laser, whose maximum output energy is not high, can produce a beam spot diameter of several millimeters at the widest. In contrast, the second method, in which the maximum output energy of a laser is very high, can produce a large spot of more than several square centimeters (in general, the beam pattern is square or rectangular), to provide high mass-productivity.
However, where a usual square or rectangular beam is used to process a single, large-area substrate, it needs to be moved in all the four directions. This point still remains to be improved.
This can be greatly improved by deforming the beam into a linear shape, making its longitudinal dimension longer than the width of a substrate to be processed, and scanning the substrate with the beam, leaving, as an item to be improved, nonuniformity of laser illumination effects.
A pulsed laser has a feature that the output energy somewhat varies between pulses. Further, the degree of the output energy variation depends on the output energy level. Therefore, when illumination is performed in an energy range where stable laser oscillation is hard to establish, it is particularly difficult to perform laser processing with uniform energy over the entire substrate surface.
An object of the present invention is to solve the problem of nonuniformity. As for a method of reducing the above-described nonuniformity, it has been reported that the uniformity is improved by performing, before illumination with strong pulsed laser light, preliminary illumination with pulsed laser light that is weaker than the strong pulsed laser light. Further, it is known that the uniformity of the substrate surface is improved by making the scanning directions, with respect to the substrate, of the preliminary illumination and the main illumination approximately perpendicular to each other because nonuniform characteristics of the two illuminating operations cancel out each other. However, even this illumination method does not address the uniformity that is caused by a temporal variation of the laser energy.
In the above method in which the same substrate is illuminated with laser beams of different energy levels, the output energy needs to be changed before each of the second and following illuminating operations. Since the output of a pulsed laser is unstable for a while after the laser output is changed, the illumination should be suspended until the laser output becomes stable. As such, while plural times of laser illumination has an advantage of improving the uniformity of the substrate surface, it has a disadvantage of greatly increasing the processing time. Another object of the invention is to solve this problem.
In the laser crystallization, the magnitude of the laser energy has large influences on the characteristics of produced semiconductor devices. Therefore, in a laser crystallization process, the optimization of laser energy is one of the most important subjects. However, the stability of the output energy of pulsed laser light tends to be greatly reduced when the laser output energy is too low. Therefore, if a desired energy level is located in a range where the laser oscillation is unstable, the uniformity of the laser illumination surface will be very bad. Another object of the invention is to solve this problem.
According to the invention, the above-described problems are solved by using a light attenuation filter. More specifically, a laser is caused to produce such output energy that the laser can operate as stably as possible, and at least one light attenuation filter is used, to thereby perform laser illumination with the laser output energy adjusted at the optimum level. The method of the invention is particularly effective in the preliminary illumination that is performed at a low energy. It goes without saying that a light attenuation filter need not to be used where the optimum energy can be produced stably without using it.
The invention is more effective if it is practiced in a method in which the same substrate is subjected to plural times of laser illumination and the laser energy is changed in every illuminating operation (for example, the above-described method in which the two illuminating operations (preliminary and main) are performed). In the conventional method, the laser output needs to be changed because the preliminary and main illuminating operations should be performed at different energy levels. However, this makes the laser output unstable, and certain time is required for the laser output to become stable. Therefore, each of the second and following illuminating operations needs to be suspended until the laser output becomes stable. The invention can eliminate this waiting time. That is, by changing the light energy only by using the light attenuation filter, i.e., without changing the laser output, plural laser illuminating operations can be performed with the laser output kept stable. Thus, there is not required time for stabilizing the laser output.
The above method of performing plural laser illuminating operations can be practiced very efficiently by reciprocating a laser beam relative to the substrate. That is, the time required for the laser illumination can further be saved. In this method, the light attenuation filter may be used such that it is quickly inserted into or removed from the laser light optical path upon completion of the go or return movement of the laser beam illumination.
The advantages of the above-described constitution are not limited to the energy adjustment by mere combination of the laser and the light attenuation filter, but include that the laser illumination can be performed stably at an arbitrary energy level that is lower than the minimum energy level that can be obtained stably by a pulsed laser, and that the laser illumination can be performed continuously without changing the laser output in the above-described method of performing plural times of laser illumination at different energy levels.